In humans, words have a lot of power. A text or an e-mail can turn a friend to an enemy, or a bad day into a good one. Words are the beginning of fights and the beginning of relationships. For people all over the world, the pen is mighty.

But for mice, there is no pen. There is only pee. A new study shows that certain pheromones called major urinary proteins, or MUPs, in male mouse urine form a unique signature that mice use to determine whether to mark or countermark territory. The results show that mouse behavior is far more complex than previously thought, and the findings provide new ways to explore mouse decision-making though the golden powers of urine.

Anyone who has confronted a mouse knows that they can produce an astonishing volume of urine. This is how male mice leave a message: Peeing on an area lets other mice know this is owned property. When another mouse comes along, he smells the urine left behind by the first. If the second male is dominant to the first male, he will leave his own yellow countermark, letting the first mouse know that his presence is not welcome.

But producing all that urine takes a lot of energy (not to mention a lot of water). In addition, the urine of male mice is full of MUPs, large pheromones that also take a lot of energy to make. Molecular neuroscientist Angeldeep Kaur, now at the University of North Carolina in Asheville, and colleagues from the Scripps Research Institute in La Jolla, Calif., wanted to look into this countermarking behavior to understand what makes a mouse pee to mark territory.

Mice can produce up to 21 different types of MUPs, but most express four to 12 types, producing a unique eau de mouse. This expression is constant throughout the mouse’s lifetime. In findings published April 24 in Cell, the scientists show that MUPs from a mouse’s own urine (or the urine of a genetically identical mouse) resulted in no countermarking. But MUPs from another mouse’s urine made a test mouse pee all over the place.

The scientists were able to show these different combinations of MUPs causes strong responses in the mouse’s vomeronasal organ, or VNO, an area that senses pheromones. Previously, scientists assumed that the VNO responded separately to each pheromone. One pheromone: one behavior. But the new study shows that in fact, the VNO responds to combinations of MUPs with different behaviors based on previous experience.

And altering the MUPs that a mouse is exposed to alters their “scent of self.” When researchers added an extra MUP to a mouse’s cage, mixing it with the normal MUP combinations from their urine, the mice lost the ability to distinguish the smell of their own urine. They began marking aggressively, defending their territories from themselves. The same thing happened when mice were housed in a cage previously inhabited by a mouse with different MUPs. After two weeks of exposure, the mice thought the old inhabitant’s MUPs were their own. But once the subjects were returned to their own cages, they again adapted to their own scent of self, and stopped countermarking.

A particular MUP signature produces a cascade of decision-making behaviors. Each time a mouse confronts another’s urine, he makes a decision: to hold it or let it go. “The mouse can sense the identity of the previous resident,” says Lisa Stowers, a molecular neuroscientist at Scripps and study coauthor. “And it can weigh its previous experience with them. Is this Joe? Am I dominant to Joe?”

While combinations of MUPs appear to produce a specific mouse signature, there are two MUPs in particular that produce a very specific response: rage. MUP3 and MUP20 didn’t make male mice countermark, it made them go for the throat of the nearest mouse. Exposure to either of these MUPs alone, or in combination with other MUPs, were enough to make male mice display aggression, causing them to attack any mouse bold enough to wear the pheromone.

MUP20 is the pheromone darcin, named for the handsome love interest from Jane Austen’s Pride and Prejudice. MUP20 might make male mice rage, but it makes female mice swoon. Stowers believes that the rage-inducing effects of MUP20 in males might be a side effect. “Even though male mice have co-opted it, it’s there for the ladies,” she explains.

The study shows that “the relationship between MUP excretion and the responding behavior depends on specific context and experience,” says Pavel Stopka, a behavioral ecologist at Charles University in Prague. It means pheromones produce more complex behaviors than scientists once believed. In mice, Stopka says, MUPs are “the key players in chemical communication.”

While the study shows that MUPs elicit more complex behaviors than previously believed, Stowers would like to use mouse countermarking behavior to study not only social interactions in mice, but decision-making behavior as well. After all, everybody pees. “It’s as simple an assay as can be,” Stowers explains. “The learning is revealed automatically, and you can study mice making decisions based on previous information.”

Further studies with MUPs could help discover the circuits that underlie how mice, and men, make decisions. The pee is mighty, indeed.